Many different requirements are expected of the network layer in all-IP access networks (e.g. 4G cellular networks). Two in particular are mobility and QoS. The former enables users to communicate seamlessly with remote network nodes via the Internet wherever they are, whereas the latter enables users to receive different levels of service for certain types of traffic. However, research has shown that problems may arise when attempting to configure an access network to operate a mobility protocol at the same time as a QoS routing protocol.
Best effort routing protocols such as Open Shortest Path First (OSPF) have been extended with QoS functionality. For example QoS Extensions to OSPF (QoSPF) (see RFC 2676) have been proposed in which the routing architecture of OSPF is augmented to include QoS-related link metrics e.g. the amount of bandwidth available at each link. Since OSPF (and therefore QoSPF) is an intra-domain link state routing algorithm, each router in the access network stores a database of the entire topology of the domain. Each router discovers its neighbouring routers and sub-networks, and advertises its local environment to other routers in the administrative scope of the network using a reliable flooding mechanism. These advertisements are stored and updated to synchronise routing knowledge in the network. The routers in the network may operate on an explicit route basis or on a hop-by-hop basis.
When operating a QoS routing algorithm it is prudent to operate some resource reservation system. For example a Bandwidth Broker may be used to admit a Reservation Request for a packet flow to travel a certain path across the access network. The Bandwidth Broker stores a database of the network topology and link state (based on the router advertisements for example). Using the database the Bandwidth Broker can decide whether or not to accept the Reservation Request. Therefore for hop-by-hop routing, although in principle the QoS route might be changed by routers on the path as new link state information is gained, this is not practical since a new Reservation Request would need to be made to the Bandwidth Broker. Accordingly, once the route is chosen for the session the hop-by-hop route does not change until a handover is performed.
Mobility at the network layer is concerned with maintaining the routability of packet data to and from a mobile node when that mobile node moves away from its home access network. The main candidate for provision of this functionality is Mobile IP (MIP). Very briefly MIP relies on a Home Agent in the home access network to tunnel IP packets to the domain where the mobile node is attached. The mobile node forms a Care-of Address (CoA) that is globally topologically correct in the network to which it is attached. The Home Agent encapsulates packets that it receives (addressed to the mobile node's home address) in another IP packet addressed to the CoA. In this way packet data may still reach the mobile node even when it is away from the home network. Further details of Mobile IP can be found in RFC 3344, 3775 and 3776 to which reference is specifically made.
However, when a mobile node hands over to a new access router, binding updates are triggered to the Home Agent, etc. These binding updates can introduce unwanted delays and loss of packets, and thereby degradation in performance from the user's perspective. When attached to a particular wireless access network (such as a cellular network), a mobile node may change its point of attachment (i.e. access router) quite frequently (e.g. every few minutes or more often, particularly if on the move). Each change triggers configuration of a new CoA, followed by the necessary binding updates. Doing this frequently (e.g. every few minutes) is not practical.
Hierarchical Mobile IPv6 (HMIPv6) has been proposed (see RFC 4140) to address this problem. HMIPv6 provides a mobility agent known as a Mobility Anchor Point (MAP) in the access network. A MAP is a logical entity that handles micro-mobility for the mobile node. Micro-mobility is a change in point of attachment of the mobile node from one access router to another, both of which are within the same domain of the access network. Whenever this happens, the mobile node sends a binding update to the MAP (comprising a new Link local CoA or LCoA), but the mobile node's primary CoA (or Regional CoA or RCoA) remains unchanged. In this way the mobile node can move between access routers in the same administrative domain without having to send a binding update to the Home Agent. In contrast when the mobile node changes point of attachment to an access router in a different access network, this is a macro-mobility event i.e. requiring a binding update to be sent to the Home Agent of the mobile node.
When an access network operates both a mobility protocol (such as HMIPv6) and a QoS routing protocol, the requirement for all packets to pass through a particular MAP in the domain breaks one QoS route (gateway to access router and vice versa) into two. In particular, due to the high volume of traffic that it handles, it is almost certain that the MAP does not lie on the best QoS route from the gateway to the access router. Even though two QoS routes are then calculated (gateway to MAP, MAP to access router), their combination is by definition not the best QoS route if the MAP does not lie on the route that would be computed between the gateway and the access router. This causes a routing conflict between mobility on the one hand and QoS routing on the other. Thus attempts to operate both tunnelling-type mobility protocols and QoS routing protocols at the same time have not produced the performance gains that might be expected.
One way to address this problem was described in our co-pending UK patent application number 0716529.3 (the disclosure of which is incorporated fully herein for all purposes). That document disclosed: for a mobile node visiting a packet-switched wireless access network, said access network using a tunnelling-type micro-mobility protocol and a Quality of Service (QoS) routing protocol to route packet data to and from said mobile node, and comprising an access router to which said mobile node may attach, a mobility agent and a gateway, a method of configuring said access network to route packet data toward said mobile node which method comprises the steps of:
(a) receiving in said access network a handover indication of said mobile node or a login request therefrom;
(b) in response to said handover indication or login request computing a QoS route for said mobile node, which QoS route is useable to route packets between said mobility agent and said mobile node;
(c) performing step (b) at a location in said access network remote from said mobility agent; and
(d) transmitting said QoS route and said handover indication or login request from said remote location toward said mobility agent;
whereby upon receipt of data transmitted in step (d) said mobility agent may handle both mobility configuration and QoS route configuration for said mobile node as part of said network layer handover. In one embodiment, one QoS route (between mobility agent and access router) was calculated by the access router, and the second QoS route (between gateway and mobility agent) was calculated by the mobility agent.
We have identified a problem that arises in a network environment comprising a plurality of such networks, each having a limited geographical area of coverage provided by wireless access points (such as one or more UMTS Node B for example). When a mobile node establishes one or more data session (such as a web browsing, streaming, or packet data call e.g. VoIP) in one network, there is some probability that the mobile node will move to the edge of the area of signal coverage of that network during whilst the session(s) is ongoing. As mentioned above, this necessitates a macro-mobility event. It is necessary to handover the sessions of the mobile node to the new network and such a handover is referred to herein as an inter-access network handover. It is to be noted that this problem is quite different from the problem of intra-access network handovers, which is specifically addressed in RFC 4140.
During an inter-access network handover full re-establishment of mobility and QoS must take place. Such a handover is possible under MIPv4 or MIPv6, but results in large delays that increase packet loss and the chances of breaking up the ongoing session(s) of the mobile node (e.g. due to timeouts of TCP connections). For example in G. Xie et al “Handover Latency if MIPv6 Implementation in Linux”, IEEE Globecom Proceedings, November 2007, it was found that handover delays could be as long as 3.6 s. This delay is only associated with Mobile IP. If the access network also supports HMIP and QoS, the delay in registering with the new mobility agent and setting up QoS routes is expected to be longer still, and certainly unacceptable from the point of view of session continuity.
The delays associated with mobility can be sub-divided into: (i) movement detection delay; (ii) CoA configuration delay including Duplicate Address Detection (DAD) and local binding update; and (iii) Mobile IP binding update delays. The first of these delays is caused by the interval that router advertisements are broadcast in the new network (e.g. every few seconds). For example, if a mobile node moves and has to wait 5 s before receiving a router advertisement, this delays the start of the handover process. The second of these delays is caused by the time taken to configure both RCoA and LCoA addresses, send them to the new MAP (or EN) in a local binding update, and for the MAP to perform DAD, and acknowledge to the mobile node that it is now registered in the new network. Only then is the mobile node able to send a Mobile IP binding update to its Home Agent and any correspondent node(s). The Mobile IP binding update and acknowledgement causes the third time delay. The sum of all these delays is expected to last anything from a few seconds to up 30 s, causing most sessions to be interrupted.
After this, further delays are expected to be caused by QoS set-up. In particular, once the mobile node has registered with the new mobility agent in the new access network, QoS has to be set up for the mobile node. If the new access network supports IntServ this can cause severe delays whilst resources are reserved along the path from the gateway to the mobility agent, and from the mobility agent to the access router. If the access network supports DiffServ, negotiation must take place with a Bandwidth Broker for admission of the session(s) of the mobile node. Furthermore all of the link-layer mappings for the mobile node must be re-established in the new access network. In particular for each session of the mobile node, the access point or the access router holds a set of QoS mappings. These mappings translate the network-layer QoS classes in the wired part of the network to the scheduling at the link and physical layers in the wireless part, and vice-versa. In this way the QoS for each session can be preserved across all links in the network.
Accordingly it is clear that delays caused by mobility and QoS set-up procedures will be problematic from the point of view of ensuring seamless connectivity from the user's perspective.
Audsin, D. P. et al. “A Combined Mobility and QoS Framework for Delivering Ubiquitous Services”, PIMRC 2008, 15-18 Sep. 2008 mentions the problem of inter-access network handover between networks of the type mentioned. It is suggested so-called Enhanced Nodes (ENs) present in each access network will communicate with each other to exchange appropriate information, effectively forming a logical link between the networks. In particular, QoS and security context information can be transferred from one network to the other, allowing for faster re-establishment of sessions in the new network. An EN is an IP router that extends the functionality of a MAP. In particular, an EN comprises a network support sub-layer which performs three main functionalities: mobility management, QoS and security. Each of these functions is logically inter-connected with a radio resource management function. Finally a signaling part of the EN enables the EN to gather information and share information the signaling other ENs.
However, it is not explained how inter-access network handovers might be initiated, nor how the Enhanced Nodes are able to communicate with one another.